Optical devices employing beam folding with polarizing splitters

ABSTRACT

A beam folder increases optical length with polarizing beam splitters and reflectors that repolarize a beam by retarding it. An incident beam encounters the beam splitters multiple times, and are both passed and rejected by the same splitters. The splitters and repolarizing reflectors can be shaped to perform optical functions in a smaller volume. Valves and controls can varyl the beam intensity and combine multiple beams. Applications include projection, imaging, collimating, mixing, and balancing.

This application is a continuation of U.S. application Ser. No.09/751,339 filed on Dec. 29, 2000 now U.S. Pat. No. 6,597,504 which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to optical systems, and more particularlyconcerns devices for folding an optical beam so as to increase its beamlength.

BACKGROUND

Many devices for processing optical beams have significant sizerestrictions, yet require relatively long lengths for the beams insidethe devices.

A common technique for increasing the length of a beam within an opticaldevice is to fold the beam inside the device. Binoculars, for example,commonly reflect incident optical beams in a “Z” shape to increase theiroptical length while keeping their physical length small. Many reflexcameras employ a pentaprism to increase the optical length of theirviewfinders within a small camera body. Projection systems of variouskinds attain wider deflection angles in a shorter distance fromprojector to screen by folding their beams internally. Projectiontelevision receivers, for example, frequently employ shaped mirrors tofold the beams traveling from the guns to the screen. Collimators andother types of devices can achieve increased beam length by foldingincident optical beams within the devices. Heads-up displays andhelmet-mounted military optics require multiple optical functions in asmall volume. Other kinds of radiation beams, such as x-rays andelectron beams, can also be processed by folding them in suitabledevices.

A number of conventional devices fold optical beams with plane mirrorsor other reflectors. Beam folders implemented with conventionalreflectors generally do not save large amounts of space. That is, atleast one physical dimension of such a device remains a large fractionof the effective optical length within the boundaries of the device.Other conventional folding devices employ beam splitters. These devicescan significantly increase optical length compared to their physicaldimensions. However, beam splitters typically suffer from low opticalefficiency. The intensity of the output beam is frequently only a smallfraction of the incident beam's strength. In adsdition, such devices donot perform other optical functions within the device. Their onlyfunction is to increase path length; any further beam manipulation mustbe accomplished separately, thus increasing the overall volume of thedevice.

SUMMARY OF THE INVENTION

The present invention offers optical devices for increased opticallength in restricted volumes using polarizing beam splitters reflectorsthat repolarize or convert the polarization of an incident beam, thusproviding greater optical manipulation of the beam in a given physicalspace. Some of the devices employ polarized beams, others operate withnon-polarized beams.

Devices according to the invention include one or more polarizing beamsplitters each having a pass axis that transmits one polarization of anincident beam through the splitter, and a rejection axis that reflects adifferent polarization from the splitter in a different direction.Devices incorporating the invention also include one or morerepolarizing reflectors, that is, reflectors that change both thedirection of an incident beam and its polarization. These elements areconfigured to transmit an incident beam entering the device among eachother so that one or more of the splitters both passes and rejects thebeam because of a polarization change in one or more of the reflectors.

The invention finds utility in optical systems for many applications,such as projectors, imagers, collimators, and manipulators of opticaland similar radiation. The terms “optical” and “light” must be taken ina broad sense as including any wavelength and type of radiated energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1.1-1.4 show optical elements for illustrating concepts used inthe invention.

FIG. 2 is a schematic diagram of a beam-folding device showing one formof the invention.

FIG. 3.1 is a schematic diagram of another form of the invention.

FIG. 3.2 shows a compound device using multiple devices of the formshown in FIG. 3.1.

FIGS. 4-9 are schematic diagrams of further forms of the invention.

FIGS. 10-11 are diagrams of polarizing beam splitters useful in theinvention.

ENVIRONMENT

FIGS. 1.1-1.4 illustrate conventional components and concepts useful indescribing the invention. A beam or ray entering a component or devicedescribed herein is an incident beam. When the beam portion leaving thedevice is called out separately, it is termed the exit beam. Beams areshown as light-weight lines in the drawing. A beam may have itsdirection, polarization, and/or other properties modified when itencounters a component within a device or a surface thereof. Whenpolarizations are indicated in the drawing arrows and circles representdifferent polarization directions or modes. These directions arearbitrary, and different symbols merely signify that the designatedpolarization modes differ from each other. For example, a beam said tobe horizontally polarized may have any direction with respect to anoptical device; the only significance of the designation is that itspolarization is opposed to a beam in the same device that is termedvertically polarized. The terms ‘S’ and ‘P’ are sometimes used for thesemodes. Polarization can also occur in mutually opposed right- andleft-hand circular and elliptic modes. The description below will focusupon beams in the optical range of electromagnetic wavelengths. Theinvention also applies to other ranges of electromagnetic radiation,such as x-rays or radio waves. It can also apply to waves of otherkinds, such as acoustic energy. Optical components such as reflectorsand splitters have analogs known to those skilled in these fields oftechnology.

FIG. 1.1 is a schematic diagram of a typical pentaprism optical devicefor increasing optical length for the viewfinder of a single-lens reflexcamera, indicated generally by the numeral 110. An incident optical beamor ray 111 enters camera lens 112 and reflects from hinged mirror 113 infront of film plane 114. The beam enters pentaprism 115 through lowerface 115.1, then encounters a reflective coating on face 115.2. The beamreflects to another face 115.3 that also has a reflective coating. Faces115.2 and 115.3 are angled so that the beam exits the prism through rearface 115.4 in a direction perpendicular to the direction from which itentered the prism. Finder lens 116 then forms a small upright image forthe viewer.

Thus, the travel distance of beam 111 is increased significantly overwhat it would be by merely reflecting beam 111 from a mirror placed at117. The pentaprism folds beam 111 so that its length inside the prismdevice is significantly greater than an associated physical dimension ofthe device. This dimension is normally a physical length or width of adevice. In prism 115, the associated dimension is the distance fromentrance face 115.1 to point 117, plus the distance from 117 to exitface 115.4. That is, the applicable dimension is the distance the beamwould have traveled within the device had it not been folded. Thus,folding makes the optical length greater than the physical dimension ofthe device. This description employs the term “beam length” to denotethe distance traveled by a beam inside the device. In optics technology,the term “path length” denotes this optical distance, but also takesinto account the index of refraction of the material through which thebeam passes. In the present context, this difference is usually small.

FIG. 1.2 is a schematic view of a conventional polarizing beam splitter120. An incident beam 121 having a polarization indicated by arrows 122is transmitted or passed through the splitter as exit beam 123 havingsubstantially the same direction as the incident beam. An incident beam124.1 or 124.2 having a different or opposite polarization, indicated bycircles 125, is rejected by the splitter—that is, reflected from it in adifferent direction, as shown for exit beam 126.1 or 126.2. Thepolarization mode that the splitter transmits or passes is called itspass axis; the mode that it reflects or rejects is its rejection axis.The pass and rejection axes of a polarizing beam splitter are generallyperpendicular to each other, although it is possible that they mighthave some other angle with respect to each other. For circularpolarization, the pass axis could be a right-hand polarization mode,while the rejection axis would be an opposite left-hand mode. That is,the term “axis” does not necessarily denote a linear or other literaldirection, and can be taken as synonymous with a polarization mode.

Polarizing beam splitters can be constructed in a number of ways, suchas with an active layer 127 providing a polarizing beam-splittingfunction on a glass or other transparent substrate 128 that providesmechanical strength. Although certain specific forms are portrayed here,any construction is useful in connection with the invention. Thesplitters described here transmit one polarization mode with itsdirection unchanged, and reflect another mode at an angle equal to itsangle of incidence. The significant point, however, is that differentpolarizations are separated into beams having different directions orlocations. Therefore, the terms pass and rejection axes denote in ageneral sense the orientations of the different effects that thesplitter has upon waves of a given polarization, and are not limited tothe transmission and reflection of different polarizations at particulardirections or angles.

FIG. 1.3 depicts one form of wave-repolarizing component 130. Such acomponent alters the phase of a wave passing through it by a certainamount, usually expressed as a fraction of a wavelength or an angle. Inthe plate shown schematically as 130, for example, incident beam 131encounters the plate with a phase shown at 132, and emerges as an exitbeam 133 with a retarded phase indicated at 134. A reference beam 135not passing through the plate is shown for comparison. The phase 134 ofexit beam 133 is 180° or one-half wavelength out of phase with respectto the phase 136 of the reference beam. Plate 130 is therefore called ahalf-wave retarder. Other phase amounts are common; quarter-wave (90°)retarders, e.g., are used in many applications. Retarders have theeffect of changing the polarization mode of an incident wave or beam. Ahalf-wave retarder, for example, converts or rotates a horizontallypolarized beam to a vertically polarized one and vice versa, whenproperly oriented. Right- and left-hand circularly or ellipticallypolarized waves are also interconverted to their opposite modes by ahalfwave retarder. Some specific forms of retarder are discussed below,but any type can be employed. Moreover, other conventional methods ofaltering the polarization of a wave can be turned to use in theinvention.

FIG. 1.4 is an enlarged partial view of one form 140 of a single opticalelement for performing two operations in the present invention. Element140, termed a reflecting repolarizer, both reflects an incident wave 141and modifies its polarization mode. Beam 141 first passes through alayer 142 that retards it by a quarter wavelength in a wavelength rangeof interest. Beam then reflects from layer 143 to form exit beam 144having an angle equal to the incidence angle of beam 141. Because thebeam passes through layer 142 twice, its total retardation is a halfwavelength, and its polarization mode is opposite that of the incidentbeam 141, the same as in FIG. 1.3. Beam 141 reflects from the frontsurface 143 of layer 145. Although a rear-surface reflector is alsopossible, front-surface reflectors are usually more efficient. Such areflecting repolarizer provides two operations required for path-lengthenhancement in the present context packaged in a single optical elementthat can be constructed and positioned as an integral unit, thus savingboth expense and space. The two operations are also performed nearlysimultaneously, so that untoward effects cannot intervene to upset theiralignment or mutual cooperation. A device according to the invention,however, can also employ two separate components for the operations ofreflecting a beam and modifying its polarization; the term “reflectingrepolarizer” must be understood to embrace any device or combination ofdevices that perform these two functions.

DETAILED DESCRIPTION

FIG. 2 shows schematically an optical device 200 for folding an incidentbeam 201 to produce an exit beam having a beam length greater than anassociated dimension 203 of the device. Light 201 from any type ofsource 210 has a polarization indicated at 204. The source could beinherently polarized, or a conventional analyzer (not shown) could beplaced in the path of incident beam 201 into or within device 200, evenat the exit. The polarization mode is arbitrary; it could be linearvertical or horizontal, circular right- or left-handed, etc. Apolarizing beam splitter 220 is positioned so that its rejection axislies along the polarization mode of beam 201. Therefore, the beam isreflected from splitter 220. The reflected beam then passes throughpolarizing bean splitter 240, whose rejection axis is perpendicular tothat of splitter 220, and encounters a reflecting repolarizer 230. Thebeam, now having a different or opposite polarization, as symbolized at205, next encounters polarizing beam splitter 240 having its pass andrejection axes positioned so that the beam is now rejected. When thebeam again encounters the first splitter 220, its polarization is nowaligned with its pass axis. Thus the beam is transmitted throughsplitter 220, and leaves device 200 as exit beam 202 directed toward atarget, shown schematically at 250. For convenience, FIG. 2 symbolizesthe pass and rejection axes of the splitters with the same polarizationsymbols used for the beam. If the angle or position of input beam 201changes, say to the right in FIG. 2, the incident beam might firstencounter splitter 240, which would pass it to splitter 220, which wouldreject it to repolarizer 230. The repolarizer would reflect the beamback to splitter 220, which would now pass it to splitter 240 to bethere rejected and sent out of the device at the top. That is, the orderof encounters can change, but the effect remains the same.

The optical length of beam 210 within device 200 is twice the physicallength 203 of the device, with no significant optical losses. Splitters220 and 240 are depicted as perpendicular to each other, and diagonal(here, about a 45° acute angle) to reflecting repolarizer 230, in thestyle of an X-cube combiner. The axes of the crossed splitters areorthogonal to each other: the rejection axis of one is the same as thepass axis of the other, and vice versa. The beam encounters eachsplitter twice, and the repolarizer once. Each splitter both passes andrejects the beam, at different points along its path. Obviously, theaxes of both splitters could be interchanged, or the beam polarizationcould be rotated. In that case, the beam would first pass throughsplitter 220, then be rejected at 240, and so on. Repolarizer 230 wouldthen be placed at the other side of the device.

Most substances employed as polarizing beam splitters are isotropic, andhave no intrinsic pass and rejection axes. Some materials, such asflexible plastic sheets sold under the name “Dual Brightness EnhancingFilm” (DBEF) by Minnesota Mining & Manufacturing Co., are inherentlyanisotropic, and can serve in the invention as polarizing beam splittersby themselves. That is, the material itself has pass and rejection axes,and does not require additional components such as half-wave retardersin order to form a polarizing beam splitter. This property significantlydecreases weight and size. DBEF film performs well in the visible-lightrange, has a large acceptance angle, and is inexpensive. Methods forpreparing it for use in the invention are discussed in connection withFIG. 10.

Device 200 might find application in a projection system, for example.Light source 210 would be a projector, and target 250 a screen. Theentire system may fit behind the image, and the losses are small.

FIG. 3.1 is a schematic representation of a device 300 having aconstruction similar to that of device 200, FIG. 2. Light source 310emits a beam 301 that is unpolarized—that is, one having both horizontaland vertical polarization portions, as indicated at 302. One portion 303is rejected at polarizing splitter 320, passes through splitter 340,reflects with opposite polarity at repolarizer 330, is rejected atsplitter 340, passes through splitter 320, and exits to target 350 at304, as in device 200. Components 320, 330, and 340 are constructed withthe same orientations as components 220, 230, and 240; that is, the passand rejection axes of splitter 320 are interchanged at splitter 340. Theother portion 305 of incident beam 301, having as polarization differentfrom that of portion 303, passes through splitter 320, but is rejectedat splitter 340. This portion then encounters another reflectingrepolarizer 360, changes polarity so that it now passes through splitter340, reflects from splitter 320, and exits to target 350 as exit beam306. Splitters 320 and 340 can be fabricated from a film havingintrinsic pass and rejection axes, or from other structures, such astilted sheets or layers having half-wave retarders (not shown).

Device 300 has many applications. It is well known that a long bar canbe used as a high-efficiency mixer of multiple beams. Device 300 willperform as a high-efficiency mixer of multiple incident beams 301 frommultiple sources 310, but has only half the physical dimension 307 of anon-folded device having an equivalent optical beam length. Ifrepolarizers at 330 and 360 are replaced with repolarizers having avariable phase change or retardation coefficient, then device 300 canfunction as a variable attenuator or dimmer. Components 371 and 372symbolize conventional variable attenuators or other controllers forthis purpose. In this case, the variable retardation only partiallychanges the polarization from horizontal to vertical or vice versa, sothat the splitters will only pass or reject a fraction of the power inthe beams that encounter them for different incident polarizations.Multiple units 300 can be placed in series to obtain a composite devicehaving any required dynamic range. With suitable control of the variablerepolarizers, the output beam could be smoothly faded from onepolarization through random polarization to another polarization. Avariable retarder such as 380 on the output face of device 300 cancontrol the polarization of the exit beam by allowing only onepolarization mode to pass. If only one repolarizer, such as 330, can bevaried, then two devices in series constitute a dimmer, but only one set371 of control electronics is required. In the limit, of course, avariable attenuator could be merely an on/off switch—a valve forselectively gating the beam in response to an electrical signal or otheragency.

Some forms of polarizing beam splitter operate effectively over only aparticular band of wavelengths. If splitters 320 and 340 operate only atone color or other range of wavelength, device 300 is useful to controlcolor balance. If, for example, reflecting polarizers are selected, theamount of red light in the beam can be controlled without affectinggreen and blue. If this form of device 300 is cascaded with additionalunits 300 having green splitters and then blue splitters, the compositedevice can perform mixing, dimming an color balancing at the same time.Cascading optical units mens to position them so that the exit beam ofone becomes the incident beam of the next.

FIG. 3.2 shows such a composite device. Individual units 300.1, 300.2,and 300.3 are cascaded, so that the exit beam of one constituent devicebecomes the entrance or incident beam of the next. Constituent devices300.1-300.3 have splitters effective over red, green, and bluewavelength bands, respectively. (Other wavelength bands could also beemployed.) Controller pairs 371.1/372.1, 371.2/372.2, and 371.3/372.3vary the retardation of the devices separately, and hence vary theintensity of each color independently. Each constituent unit 300.1,300.2, and 300.3 transmits two of the three colors as if there were noreflective polarizers—that is, as if their splitters had no rejectionaxes.

The configuration of device 300′ causes the rays to bounce off thereflecting repolarizers of the constituent units at steep angles. It isdifficult to obtain high efficiency at both near-normal and largeincidence angles. If each unit is constructed of a solid (this termincludes a liquid) transparent material having a high refractive index,a low-index coating placed between the solid material and eachrepolarizer causes the unaffected portions of the beam to be reflectedby total internal reflection, a much higher-efficiency mode than grazingreflections. This technique is useful in other embodiments also, and isparticularly desirable in devices employed as mixers where some of theincident radiation is not affected by the splitters. If the refractiveindex of the repolarizers is sufficiently low, then no coating may beneeded to achieve total internal reflection. The mirrors of therepolarizers can then be optimized for reflections at near-normal anglesat the wavelengths of interest. Depending upon the size and angularextent of source 310, and the number of units in series, a large numberof large-incidence angle reflections may occur from the repolarizers; alow-index coating will increase the efficiency of these systems as well.

Tilted transparent plates in an optical system introduce aberrationswhen beams traverse them at an angle. The aberration that typically hasthe greatest effect upon image quality is astigmatism. In a compositesystem 300′ where multiple constituent units are cascaded, as in FIG.3.1, astigmatism can be reduced by orienting the constituent devices ina certain way, as described below in connection with FIG. 4.

The devices shown in FIGS. 2-3 have two polarizing beam splitterspositioned orthogonally to each other. Employing other numbers of suchbeam splitters, and/or configuring the beam splitters non-orthogonallyto each other, opens up further ways for performing optical functions ina reduced volume. FIG. 4 illustrates a device 400 where the incidentbeam 401 from source 410 is limited to any predetermined polarization,such as vertical or right-hand circular. A single polarizing beamsplitter 420, constructed like splitter 320 or 340, FIG. 3.1, has itsrejection axis aligned with the polarization of beam 401. Thus, the beamreflects from the splitter and encounters repolarizer 430. The beam,having had its polarization changed by element 430, now passes throughsplitter 420, and impinges upon another repolarizer 440. The beam, nowmodified back to its original polarization direction, is thus reflectedagain from splitter 420, and exits the device. That is, the beamencounters splitter 420 three different times, being rejected twice andpassed once. The two repolarizers are positioned parallel to each otherand diagonally to the splitter, so the exit beam 402 leaves in the samedirection as incident beam 401 had entered. Here, the beam length isthree times the associated dimension 403, the physical length of thedevice. The modified exit beam 402 departs device 400 in substantiallythe same direction as the incident beam 401 had entered it.

Other devices, such as 400, can be compounded in the same way as shownin FIG. 3.2. Diagram 404 indicates that the YZ plane is in the plane ofthe page of FIG. 4. Let the surface normal of splitter 420 be parallelto vector j+k in the standard notation of diagram 404. Orienting thecorresponding splitters of further devices (not shown) in the samedirection compounds astigmatism through the overall device. However,orienting the splitter of a second device such that its surface normalis parallel to vector i+k (or, equivalently, parallel to −i+k) cansignificantly decrease the overall amount of astigmatism. Another way tovisualize this process is to rotate the second device by 90° in eitherdirection around the Z axis in diagram 404. For hollow componentdevices, the splitters add astigmatism; cascaded systems employing suchdevices preferably add them in pairs to cancel the astigmatism.

FIG. 5 depicts a device 500 for projecting images from a source 510 ontoa target 520. Numerals 501.1, 50.2, and 501.3 symbolize an incident beam501 of polarized rays from a liquid-crystal display (LCD) 511 or othertype of conventional light source 510. Source 510 may include projectionor focusing optics such as a projection lens 512 and a folding mirror513. Lens 512 and mirror 513 are conventional, and may be replaced byother optical components in any convenient arrangement or omittedaltogether, depending upon the desired overall configuration of thedevice 500. In this example, for instance, mirror 513 reduces theoverall size of projector 500 by utilizing the otherwise wasted spacebehind element 530.

The polarized incident beam 501 proceeds from mirror 513 of source 510to a first polarizing beam splitter 520 having a pass axis positionedwith respect to the polarization of beam 501 so as to transmit rays501.1-501.3 to a second polarizing beam splitter 540. Splitter 540 has arejection axis aligned in the same direction as the pass axis ofsplitter 520, or equivalently, a pass axis substantially perpendicularor crossed with respect to the pass axis of splitter 520. Thus, the beam501 that was passed by the first splitter 520 is reflected from thesecond splitter 540. Splitters 520 and 540, and other splittersdescribed below for other embodiments, can be constructed of sheets orlayers of material having intrinsic pass and rejection axes, or may befabricated in other ways, as described elsewhere.

Beam 501 then proceeds to a reflective repolarizer 530, which convertsits polarization to an opposite mode: horizontal to vertical, etc.(Again, polarization directions or modes are named arbitrarily; onlytheir difference is significant to the invention.) Beam 501 now has apolarization aligned with the rejection axis of splitter 520, whichtherefore reflects it back toward splitter 540. Because the axes ofsplitter 540 are crossed with respect to those of splitter 530, thepolarization of beam 511 is now aligned with the pass axis of splitter540. Splitter 540 thus transmits the beam as an exit beam 502 to atransmissive target projection screen 550. Rays 502.1-502.3 correspondto source rays 501.1-501.3 respectively. The planes of splitters 520 and540, extending out of the page of FIG. 5, are at an acute, non-parallelangle with respect to each other. The particular angle depends upon thedesired overall geometry of the system.

The projection optics and configuration of the components is such thatthe exit beam is focused to form an image upon screen 550 of the imagefrom LCD 511. FIG. 5 shows screen 550 in essentially the same locationas the second splitter 540, and is viewed from direction 551. Ifdesired, screen 550 and splitter 540 can be fabricated as a singleintegrated unit for easier mechanical support. Alternatively, they canconstitute separate physical units, or the screen can be separated fromsplitter 540 and made reflective, so that it is viewed from a directionopposite that of arrow 551. Device 500 illustrates a configurationaccording to the invention in which the polarizing beam splitters arenot perpendicular (i.e., non-orthogonal) to each other, and in which therepolarizer is positioned non-diagonally diagonal (i.e., not at 45°)with either of them. The beam traverses both splitters twice, and isboth passed and rejected by both splitters. The folded beam length isagain considerably greater than an associated dimension 502 of thedevice. Splitters for device 500, and for the devices described below,can be formed of a material having intrinsic pass and rejection axes asmentioned above, or from any other single or composite structure thatserves as a polarizing beam splitter in the wavelength band of interest.

The systems shown in FIGS. 2-5 have polarizing reflectors that areparallel and planar. Removing one or both of these requirements,however, opens up the possibility of performing other optical functionsin a physically smaller device by incorporating them into a beam folderaccording to the invention. This can be achieved by shaping thecomponents so that the same component performs an optical function orbeam manipulation in addition to simple reflection and polarizationconversion.

FIG. 6 shows a device 600 useful as an in-line collimator. Radiationsource 610 may include an illuminator depicted schematically at 611, acontrollable light valve 612, and lenses, filters, or other opticalelements 613. Rays of polarized incident beam 601 enter the device atdifferent angles. As in device 200, FIG. 2, the incident beam isrejected by a first polarizing beam splitter 620. (Some of the rays arefirst passed by a second polarizing beam splitter 640 having its axescrossed with respect to splitter 620.) Element 630 reflects beam 601 andretards it by a total of a half wavelength. This repolarizer, however,is not flat or planar, as were those described earlier. Rather, it has anon-planar shape for manipulating the beam characteristics, by focusing,collimating, or other optical operations. In this example, a parabolicshape of repolarizer redirects all of the incident rays in the samedirection, to collimate them. That is, the manipulation takes place inthe same optical element as the reflecting and repolarizing operation,and substantially simultaneously therewith, thus combining severaloperations in a single element for space and cost savings. Having beenpolarized in a different mode, the beam 501 is now rejected by splitter640, although some of its rays are first passed by splitter 620.

Rays 602 travel parallel to each other to target 650. Optional opticalelements 660 may control or further manipulate exit beam 602 if desired;for example, 660 could denote a light valve. In the illustrated device600, repolarizer 630 is shaped to achieve collimation of the exit beam.Other effects are possible with other shapes. Of course, a beamtraveling in an opposite direction through the device will be affectedinversely: parallel incident rays 602 proceeding from a source at thelocation of target 650 would be brought to a focus at the erstwhilesource 610. Repolarizers can be replaceable or controllable fordifferent effects in the same overall device. Repolarizer 630 can beconstructed as a separate element of a hollow-cube design, or the faceof a solid cube could be made with the appropriate curvature and thencoated with retarding and reflective layers. Optical elements such as612 and 660 can be constructed as separate elements or possibly byforming the entrance and exit faces of a solid cube carrying theelements 620-640. Such elements could be provided to control aberration,for example. Other optical elements, not shown, could further be placedin front of the shaped reflecting repolarizer 630, in the path betweenit and the splitters 620 and 640. The beam length of device 600 issubstantially twice its associated physical dimension 603, thusachieving collimation in a much shorter distance from source 610.

FIG. 7 shows another curved reflecting repolarizer, for applicationssuch as a head-mounted display. In device 700, an incident beam 701 fromsource 710 has a polarization aligned with the rejection axis of apolarizing beam splitter 720 and with the pass axis of anotherpolarizing beam splitter 740. The beam may encounter either splitterfirst, but encounters both before encountering reflecting repolarizer730. In this example, element 730 includes a light valve 731 that can becontrolled to alter the retardation of a wave that encounters it. Forexample, valve 731 might be settable by an electrical signal orotherwise to retard an incident beam by a half wavelength or not at all,or in gradations. One advantage of this configuration is that radiationwhose polarization is not changed by valve 731 is returned to source710, through splitters 720 and 740, along paths 702 and 703. If thesource scatters some of this radiation back without preserving itspolarization, overall efficiency of device 700 is increased. The opticalbeam length of device 700 is about three times the associated unfoldeddimension. In FIG. 7, this unfolded dimension is the length of incidentbeam 701 from source 710 to the splitter 720 that first rejects it, plusthe length of the exit beam 704 from that splitter to the exit face.

Radiation whose polarization is changed in repolarizer 730 is nowaligned with the pass axis of splitter 720 and with the rejection axisof splitter 740. This radiation proceeds to reflecting repolarizer 750,which again changes it polarization. Repolarizer 750 is shown as curved,so that it manipulates the beam in the manner of element 630, FIG. 6,although perhaps with a different result. Radiation then proceeds backto splitters 720 and 740, where it passes through the one that rejectedit previously, and is rejected by the one that previously passed it.Exit beam 704 leaves the cube 700 to target 760.

FIG. 8 shows another device 800 that finds application as an in-linecollimator. In this example, a polarized incident beam 801 from source810 is aligned with the rejection axis of polarizing beam splitter 820.The axes of splitters 820 and 840 are crossed, so that beam 801 mightencounter and pass through splitter 840 before being rejected insplitter 820. After being reflected from splitter 820, the beamencounters reflecting repolarizer 830. Element 830 is constructed as aretroreflector, so that the beam is reflected in a directionsubstantially opposite the direction it had arrived; retardation isaccomplished with a separate layer 831. Having been retarded so as tochange its polarization mode, however, the beam is now rejected atsplitter 840 and passed by splitter 820 (perhaps in reverse order). Exitbeam 802 passes from the cube to target 850. Retroreflectors can beconstructed as a single comer reflector, an array of comers, or in otherconventional configurations.

Device 800 as thus far described forms a real image in a compact opticalsystem. The optical beam length is double the associated length 803 ofthe device. If source 810 includes a collimating lens 811 and a stop812, an image of the stop is formed on target 850. Such a system offersa large eye relief in a head-mounted display, for example.

FIG. 9 is an example of a projection device 900 that demonstrates how alarge number of functional optical elements can be packed into a smallvolume using beam folding according to the invention. Polarized light901 from source 910 is rejected from polarizing beam splitter 920,possibly after passing through splitter 940. The pass and rejection axesof these splitters are crossed, or disposed in different directions.This light encounters curved repolarizer 930, a second-surface mirrorthat manipulates the beam and reflects it back to the beam splitters inan opposite polarization. If the two faces of element 930 have differentcurvatures, even further optical manipulation is achieved. On this leg,polarization is aligned with the pass axis of splitter 940 and therejection axis of splitter 920. Thus the beam passes to repolarizer 950,which is another second-surface mirror having differently curvedsurfaces. The beam is now aligned with the pass axis of splitter 940,but is rejected upon reaching splitter 920. After passing throughanother lens 960, the exit beam 902 encounterers target 970, which canbe a projection screen.

FIG. 10 is a diagram 1000 showing the use of a flexible film 1010, suchas the previously mentioned DBEF, as a polarizing beam splitter. Theactive surface of film 1010 has small ripples, reducing the resolutionof an imaging system, and thus limiting usefulness of the film forhigh-performance devices. Heating the film and placing it under tensiongreatly reduces the size of the ripples, improving its performancesufficiently for use in optical folding devices that employ the film asan imaging element. The film can be stretched in a number of ways. InFIG. 10, heated film 1010 is held over frame 1021 while frame 1022 isforced over it to produce a seal. Frames 1020 can have shapes other thanround, such as rectangular. For some shapes, the film should be cut torelieve areas of large stress, to avoid breakage. Frames 1020 stretchthe film in both directions of its plane. In the particular example ofDBEF film, the ripples are generally oriented in one direction. If theframes stretch the film only in this direction, performance is stillsignificantly enhanced.

After film 1010 has been stretched, the ripples must be maintained attheir smaller size. If the film remains in frames 1020 under tensionafter it cools, the ripples remain small. Another approach is tolaminate the stretched film to a substrate with an adhesive. The filmcan be laminated on only one side, or on both sides for added strength.Alternatively, a substance such as epoxy can fill a cavity on one orboth sides of the film; the epoxy thus provides the structural supportnecessary to maintain the curvature. Flat splitters can also befabricated by these methods.

Polarizing beam slitters can be curved as well as flat. Dashed lines1011 and 1012 indicate the curvature of a sheet. The curvature can beproduced by increasing the air pressure, symbolized by arrow 1030 on oneside of sheet 1010 while it is held in frame 1020; pressure could alsobe decreased for curving the film in the opposite direction. The shapeof the frames can be configured to determine the properties of thecurvature. Round frames produce rotationally symmetric curves, whileother frame shapes can produce curvatures having other symmetries.Curved splitter shapes can also be achieved by stretching a suitablefilm over a solid convex or cylindrical surface having the desiredcurvature.

FIG. 11 shows another polarizing optical beam splitter 1100 useful forconstructing devices according to the invention. In some applications,it is desirable to fabricate the device as a hollow air-filled cube.This frequently requires that the polarizing beam splitters be supportedon a substrate, such as transparent substrate 128 However, tiltedtransparent plates introduce aberrations when beams traverse them at anangle, as depicted in FIG. 1.2. The aberration that typically has thegreatest effect upon image quality is astigmatism. Although incidentrejected beam 124.1 is reflected directly from active layer 127, arejected beam 124.2 approaching from the opposite direction must travelthrough the substrate twice, suffering a jog with each traversal. Asubstrate having enough mechanical strength is typically at least threetimes as thick as the coating or other layer that provides the activebeam-splitting function, making the aberrations significant in manyapplications.

Splitter 1100 in FIG. 11 has a body 1110 made up of a transparentsubstrate 1111, such as glass, having an active layers 1112 and 1113 oneach side. The substrate provides sufficient mechanical strength to holdthe active layers in place. The pass and rejection axes of the twolayers are aligned with each other, so that a polarization directionthat is transmitted through either layer is also transmitted through theother; likewise, any polarization that one layer rejects is alsorejected by the other. An incident beam 1101 having a polarization(shown by the arrows) aligned with the pass axis of layers 1112 and 1113is transmitted through them and through transparent substrate 1111 asexit beam 1102. An incident beam 1103.1 having a polarization (indicatedby the circles) aligned with the rejection axis of layer 1112 reflectsit to form exit beam 1104.1. An incident beam 1103.2 having the samepolarization direction or mode as beam 1103.1, but arriving from theother side of splitter 1110, is aligned with the rejection axis of layer1113, and reflects from it as exit beam 1104.2. Therefore, neither ofthe beams 1103.1 and 113.2 need travel through the substrate 1111.

CONCLUSION

The above description is sufficient to allow those skilled in the art topractice the invention. Changes can be made in the structure, materials,and other aspects of the described embodiments without departing fromthe inventive concept, whose scope is to be measured only by thefollowing claims and their equivalents. The elements of the claims neednot be interconnected except as explicitly stated or as necessary toperform their function. Operations need not be performed in anyparticular order unless a specific sequence is recited or inherent.

1. An optical device comprising: a first polarizing beam splitter havingfirst pass and rejection axes, and positioned to receive an incidentbeam, a second polarizing beam splitter having second pass and rejectionaxes aligned respectively with the first rejection and pass axes, andpositioned at an acute angle to the first splitter; a repolarizingreflector for interconverting a polarization of the beam between thepass and rejection axes of the beam and positioned so that both beamsplitters encounter the beam at least twice; a source including afocusing element for the incident beam, where the source furtherincludes a folding mirror for reflecting the beam to the first beamsplitter.
 2. The device of claim 1 where the incident beam is polarized.3. The device of claim 1 where the screen is positioned at the locationof the second beam splitter.
 4. An optical device comprising: a firstpolarizing beam splitter having first pass and rejection axes, andpositioned to receive an incident beam, a second polarizing beamsplitter having second pass and rejection axes aligned respectively withthe first rejection and pass axes, and positioned at an acute angle tothe first splitter; a repolarizing reflector for interconverting apolarization of the beam between the pass and rejection axes of the beamsplitters, and positioned so that both beam splitters encounter the beamat least twice; a projection screen positioned to receive the beam afterit has encountered both of the beam splitters twice.
 5. The device ofclaim 1 where the screen is positioned at the location of the secondbeam splitter.
 6. The device of claim 4 where the incident beam, thesplitters, and the reflector are all positioned on the same side of thescreen.
 7. The device of claim 4 further comprising a source forproviding the incident beam, the source being configured to project animage upon the screen.
 8. The device of claim 4 where the incident beamis polarized.
 9. The device of claim 4 further comprising a sourceincluding a focusing element for the incident beam.
 10. An opticaldevice comprising: a first polarizing beam splitter having first passand rejection axes, and positioned to receive an incident beam, a secondpolarizing beam splitter having second pass and rejection axes alignedrespectively with the first rejection and pass axes, and positioned atan acute angle to the first splitter; a repolarizing reflector forinterconverting a polarization of the beam between the pass andrejection axes of the beam splitters, and positioned so that both beamsplitters encounter the beam at least twice, where the repolarizingreflector is positioned non-diagonally with respect to at least one ofthe beam splitters.
 11. The device of claim 10 where the incident beamis polarized.
 12. The device of claim 10 further comprising a sourceincluding a focusing element for the incident beam.
 13. The device ofclaim 10 where the screen is positioned at the location of the secondbeam splitter.
 14. A method for projecting an image, comprising:transmitting an incident beam from a source through a pass axis of afirst polarizing beam splitter; reflecting the beam from a rejectionaxis of a second polarizing beam splitter positioned nonparallel andnonorthogonally with respect to the first beam splitter; reflecting andrepolarizing the beam; reflecting the beam from a rejection axis of thefirst polarizing beam splitter; transmitting the beam through a passaxis of the second polarizing beam splitter to a screen.
 15. The methodof claim 14 where the reflecting and repolarizing are performed at thesame time.
 16. The method of claim 14 where the pass and rejection axesof the first beam splitter correspond respectively to the rejection andpass axes of the second beam splitter.
 17. The method of claim 16 wherethe reflecting and repolarizing are performed at the same time.
 18. Themethod of claim 16 where the first and second beam splitters arepositioned at an acute angle to each other.
 19. The method of claim 14where the first and second beam splitters are positioned at an acuteangle to each other.
 20. A method for projecting an image, comprising:transmitting an incident beam from a source through a pass axis of afirst polarizing beam splitter; reflecting the beam from a rejectionaxis of a second polarizing beam splitter positioned nonorthogonallywith respect to the first beam splitter; reflecting and repolarizing thebeam; reflecting the beam from a rejection axis of the first polarizingbeam splitter; transmitting the beam through a pass axis of the secondpolarizing beam splitter to a screen, where the operations are performedin the sequence listed.
 21. The method of claim 20 where the reflectingand repolarizing are performed at the same time.
 22. The method of claim20 where the first and second beam splitters are positioned at an acuteangle to each other.